1. Field of the Invention
The present invention relates to an FM stereo receiving system, and, more particularly, to an adaptive control for combining FM stereo signals received from multiple antennas.
2. Description of the Related Art
Traditionally, vehicles have been equipped with a mast antenna for the receipt of radio signals. Mast antennas are viewed as having shortcomings; therefore, efforts have been made to discover viable alternatives to the mast antenna. With regard to its shortcomings, the mast antenna may interfere with the desired styling of the vehicle; is susceptible to damage as when the vehicle passes by a low branch, by an act of vandalism, or by a simple accident; and has limitations in terms of its reception quality.
To address the issue of a mast antenna interfering with the vehicle's styling, antennas have been placed within the vehicle's glass, such as the windshield. Considering performance limitations, a single antenna, whether a mast antenna, an in-glass antenna, or other type of antenna, is generally susceptible to fading and multipath signal interference resulting from an obstruction such as might be caused by the presence of a building, a mountain, another vehicle, or the like. Alternatives to the whip antenna such as "in glass" antennas are typically somewhat more susceptible to fading and multipath due to their gain, directivity and polarization properties. Therefore, as alternative antennas are introduced there is increased need for other means of enhancing reception. Several techniques have been developed using multiple antennas for receipt of the radio signals to reduce the effect of such fading and interference. These techniques include scanning/selection, equal-gain combining, and maximal-ratio combining.
The scanning/selection technique is one which operates on the premise that if one antenna disposed on the vehicle is receiving a poor signal, another antenna spaced from the first antenna may be receiving a better signal. In scanning/selection systems, only one antenna is used for receipt of the signal at any particular point in time. The system either compares the signals received by the system's antennas to ascertain which antenna is receiving the better signal, or the system evaluates the signal being received to determine the quality of the signal and simply switch to another antenna if the current signal is designated as unacceptable. Though scanning/selection systems are generally an improvement over a single antenna system, the improvement is less than that obtained by more sophisticated methods. Also, the switching transients caused by switching between antennas can be audible under some circumstances. Further, because only one antenna is used at a point in time, the system provides only marginal improvement during fringe reception.
The equal-gain combining technique combines signals received by the antennas of an antenna array by correcting for the phase differences between antennas then adding the signals vectorially. No adjustments are made to the signals for any difference in the gains of the input signals. Because only the phases of the input signals are adjusted for alignment in an equal-gain system, it is possible that signal to noise may be less than optimal. For example, if two inputs are combined and one of those inputs contain mostly noise, the combined signal is likely to be of lower quality than the single non-corrupted signal.
The maximal-ratio combining technique is a further improvement over the equal-gain combining technique in that, in addition to adjusting of the input signals according to the detected phases thereof, the magnitudes of the input signals are also adjusted to yield the maximum signal-to-noise ratio. In this manner, a signal which is corrupted with noise does not degrade the overall performance of the system. Though the maximal-ratio combining technique results in an improved signal over that of the equal-gain combining technique, the cost of implementing such a system is prohibitive in some environments. The hardware complexity is typically that of having complete multiple receivers plus the combined algorithm.
In practice a switched diversity system is fairly easy to implement but offers limited improvement. The maximal-ratio combining technique offers the most improvement but at very high hardware cost. The equal-gain combining technique offers system performance just below that of the optimal-ratio technique. This is a good compromise position, particularly if we can develop techniques which have efficient hardware implementation methods, and allow combining the signals at RF or early in the IF stream. This approach is the key to efficient implementation by minimizing the excess hardware required.
In the early 1960's, an equal-gain combining technique was developed which permitted phase alignment at the radio frequency (RF) (Lewin, "Diversity Reception and Automatic Phase Correction," Proc. of IEEE, Paper No. 3584E, Vol. 9, Part B., No. 46, page 295-304, July 1962). In Lewin, a phase changer was disclosed for use in a an adaptive system. The phase changer both sensed and corrected phase. Specifically, phase perturbation is introduced and the resulting amplitude modulation is detected. Based on the work of Lewin, others developed similar techniques for amplitude modulated (AM) receivers (Parsons et al., "Space Diversity Reception for VHF Mobile Radio," Electronic Letters, Vol. 7, No. 22, pages 655-56, Nov. 4, 1971). For frequency modulated (FM) receivers, a related technique was developed (Parsons et al., "Self-Phasing Aerial Array for F.M. Communication Links," Electronic Letters, Vol. 7, No. 13, pages 380-81, Jul. 1, 1971). In the system described in Parsons, amplitude perturbation is introduced which results in phase modulated components of the sum signal which are proportional to the relative phases of the input signals. This phase perturbation is then detected and used in a feedback loop to control phase shifters and bring the input signals into phase alignment. Of course the perturbation frequency must be outside the modulation bandwidth to avoid interference with a legitimate FM signal.
To date, application of the perturbation equal-gain combining technique has been limited to systems with a simple modulation spectrum such as AM and monophonic FM. This is due to the requirement that the perturbation frequencies be placed such that they do not introduce artifacts into the systems modulation band, and that the system's modulation does not interfere with the perturbation system. The technique is advantageous over the scanning/selection technique for reasons previously discussed herein, and is much less costly to implement than are systems using the numerous maximal-ratio techniques but there are practical limitations to its usefulness. It is therefore desirable to provide a similar perturbation equal-gain combining technique applicable to complex multi-spectra communication systems, such as FM stereo.
It will be appreciated that placement of the perturbations introduced in a multi-spectra communication system such as FM stereo is more complex than in a single spectrum system. The potential for interference caused by the perturbations and any harmonics or intermodulation products generated therefrom is greater in a multi-spectra communication system. For FM, the perturbations should not cause interference with the FM monophonic band, the stereo bands or other communication bands such as radio data system information transmitted at frequencies greater than that of the FM stereo band. The challenge is to locate the perturbation frequencies in a position on the frequency spectrum where interference will not occur.